Common daytime sights normally comprise a series of optical components, including at least an eyepiece and an objective lens, acting in a similar fashion to a telescope, for magnifying the target. Adjustments to the sight include an eyepiece adjustment, normally in the form of a diopter ring, for the accommodation of different users, eyesight, and a boresight-rifle setting control, consisting of knobs, for one-time alignments, along the X axis and the Y axis, between the line of sight and the rifle's barrel.
Nighttime sights necessarily include an image intensifier for generation of an illuminated image of the target, which would otherwise appear vague or even invisible due to the lack of ambient light.
Hence, a day/night sight requires two, at least partially separate image beam, channels, one for daytime and the other for nighttime use thereof. In order to save cost and lower the weight of the unit, it is common practice to reduce to a minimum the employment of optical components, particularly regarding the eyepiece and the objective lens--required for both modes of use--by combining portions of the daytime and nighttime image beam channels.
Various constructions of unitary-body, dual-function sights have been designed. Such constructions normally included a co-axial linear arrangement of an eyepiece and an objective lens, and a parallel combination of an optical relay and an image intensifier interposed therebetween. The parallel combination is mounted on a sliding mechanism wherein in a first position, the sight is prepared for nighttime use by aligning the image intensifier along the axis between the eyepiece and the objective, and in a second, shifted position, the sight is prepared for daytime use, by slidingly displacing the image intensifier and. replacing same by the optical relay.
Sights of this type suffer from numerous disadvantages, mainly attributable to inherent deficiencies in the required sliding mechanism. Thus, special and costly design solutions have had to be used to maintain the continued fine alignment required between the sight's optical components in spite of wear on the moving parts following prolonged use, as well as providing sliding contacts for the supply of electrical power to the image intensifier. From another aspect, the prior art devices were mostly equipped with a fixed aiming reticle, which interfered with or obstructed vision of the target at the center of the field of view.